Slot die for producing webs of a foamed plastic material

ABSTRACT

A slot die for producing webs of a foamed plastics material, into which die plastics material which has been plasticized under elevated pressure in an extruder and charged with a blowing agent enters, and through the die gap of which the plastics material is extruded by foaming. The die gap is formed by a pair of rollers mounted in a laterally sealed manner in the slot die. This achieves the effect that an advantageous formation of outer layers already occurs ahead of the die gap before the start of foaming.

This is a U.S. national stage of application No. PCT/DE99/01445, filedon May 7, 1999. Priority is claimed on that application and on thefollowing application: Country: Germany, Application No: 198 26 363.5,Filed Jun. 12, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a slot die for producing webs of a foamedplastics material, into which die plastics material which has beenplasticized under elevated pressure in an extruder and charged with ablowing agent enters, and through the cooled die gap of which theplastics material is extruded by foaming.

2. Discussion of the Prior Art

DE 17 79 374 has already disclosed, for the extrusion of articles offoamed thermoplastic material, having the slot die open directly into acalibrating device, which has pressure locks in which calibratingrollers are arranged. This is intended to achieve the effect that theextruded articles can be calibrated very exactly, for which purpose itis necessary to control the pressure in the pressure chamberscorrespondingly.

U.S. Patent No. 2,857,625 has also already disclosed arranging a cooledpair of rollers behind the die gap of a die. Here, however, free foamingof the plastics material already takes place before entry into theroller nip, since atmospheric pressure prevails in the space between.the die gap and the pair of rollers.

The known solutions have the disadvantage that, after leaving the diegap of the slot die, the plastics material foams before compressed outerlayers are able to build up on both sides of the web, with the resultthat blowing agent can escape prematurely. This is disadvantageous forintensive foaming of the web. Added to this is the fact that thetransverse expansion causes waves to form in the web between when itleaves the slot die and when it reaches the roller nip. In particularwhen alternative low-boiling blowing agents or blowing agents such asfor example N2 and O2 are used, it is also necessary to work with higherblowing-agent pressures in the slot die. To ensure acceptable foamingbehavior in this case, it is also necessary to cool the plasticsmaterial at the die lips of the slot die so intensively that outerlayers are formed, in order in this way to prevent uncontrolled foamingof the emerging material. However, limits are prescribed here for thecooling, since the necessary intensive cooling may lead to the plasticsmaterial stagnating in the die lips. This causes an undefinedconstriction of the die gap, whereby the pressure and the flow in thedie gap are adversely influenced.

SUMMARY OF THE INVENTION

The invention is based on the object of avoiding the disadvantagesmentioned and of providing a slot die with which it is possible toproduce webs of foamed plastics material accompanied by continuousfoaming of the material.

This object is achieved according to the invention by the features ofclaim 1. Further development of the invention can be taken from thesubclaims. Pursuant to this object, and others which will becomeapparent hereafter, one aspect of the present invention resides in thedie zap being formed by a pair of rollers mounted in a laterally sealedmanner in the slot die.

The fact that the die gap is formed by a pair of rollers mounted in alaterally sealed manner in the slot die achieves the effect thatcompressed outer layers of relatively high-viscosity plastics materialalready form on the emerging plastics material when it comes intocontact with the surfaces of the rollers, as a result of cooling down,ahead of the die gap, i.e. before the start of the foaming operation.Since the pairs of rollers are rotating, the effect is obtained that theouter layers that are in contact are moved by the rollers of the pairsof rollers through the die gap, without stagnation and/or freezingoccurring. Moreover, the plastics melt flowing onto the rollers alwayscomes into contact with new regions of the surfaces of the rollers,which positively influences the temperature control of the melt. Thereis consequently the possibility of intensively cooling the emergingmaterial, so that a formation of outer layers already occurs ahead ofthe die gap before the start of foaming. When it subsequently emergesfrom the die gap, foaming of the plastics material then takes place, butthe outer layers keep the blowing agent in the foam core, so thatintensive internal foaming of the web occurs. Consequently, highlyfoamed materials can be produced. In addition, the outer layers preventwaves from forming in the web, since transverse expansion cannot occur.

According to a preferred embodiment of the invention, the pair ofrollers is provided on each of the two sides with a sealing thread,acting as a melt seal, as a lateral sealing means. The sealing threadmay act either as a return conveying thread for the extruded melt orelse as a sealing thread of an active melt seal, a sealant then beingfed in from the outside in the sealing thread. The sealing threadensures that no pressure drop takes place ahead of the die gap while theslot die is in operation.

It may also be provided within the scope of the invention that therollers of the pair of rollers are of a frustoconical design and aredisplaceable axially with respect to each other. Such a design of therollers makes it possible to set the thickness of the die gap by mutualaxial displacement. The axial adjustment of the rollers is made possiblein particular also by the sealing thread provided according to theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment is described in more detail below and isrepresented in the drawing, in which:

FIG. 1 shows a cross section through a slot die in a schematicrepresentation;

FIG. 2 shows a plan view of the slot die according to FIG. 1; and

FIG. 3 shows a front view of a pair of rollers of the slot die accordingto FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawing, 1 denotes a slot die, into the inflow channel 2 of whicha melt prepared in an extruder and charged with blowing agent can beintroduced. Arranged in the slot die 1 on the outlet side is a pair ofrollers 3, which is formed by two rollers and forms a die gap of theslot die 1.

As can be seen from FIGS. 2 and 3, the rollers of the pair of rollers 3are laterally provided in each case with a sealing thread 5. The sealingthreads 5 are closely enclosed in their sealing region in a way knownper se by recesses in the housing of the slot die 1. Moreover, therollers of the pair of rollers 3 are held in the slot die 1 in bearings6. The rollers of the pair of rollers 3 may be both of a cylindricaldesign and, as represented in FIG. 3 of the drawing, of a frustoconicaldesign in their region forming the die gap 4. The frustoconical designof the rollers of the pair of rollers 3 makes it possible to change thethickness of the die gap 4 and adapt it to the requirements of theextrusion by an axial adjustment of the rollers of the pair of rollers3. The rollers of the pair of rollers 3 are provided, moreover, with acentral channel 7, into which a coolant or a temperature-control mediumcan be introduced.

In the slot die 1, the blowing-agent-containing melt introduced into theslot die 1 via the inflow channel 2 reaches the inner side of therotating rollers of the pair of rollers 3. On the surface of therollers, the cooling down then causes a formation of compressed outerlayers of relatively high-viscosity plastics material on. the plasticsmelt. The rotation of the rollers of the pair of rollers 3 makes theouter layers of the melt that are in contact be discharged together withthe melt through the die gap 4, without any stagnation of melt occurringhere. In this way, outer layers are formed at an early stage, pass thedie gap 4 without being damaged and, when they emerge from the die gap4, have the advantageous effect that the blowing agent cannot escapefrom the web during the foaming of the melt. Consequently, intensiveinternal foaming of the web occurs, so that altogether an optimallyfoamed product is obtained. The outer layers additionally prevent wavesfrom forming in the foamed web, since any transverse expansion issuppressed.

The sealing threads 5 arranged on the rollers of the pair of rollers 3prevent a pressure drop ahead of the die gap 4 of the slot die 1. Theholder 8 provided in the housing of the slot die 1 is securely sealed inthe region of the sealing thread 5. Alternatively, it is possible tofeed rear-sealing melt to the sealing thread 5 in a way known per se.

Thus, while there have been shown and described and pointed outfundamental novel features of the present invention as applied to apreferred embodiment thereof, it will be understood that variousomissions and substitutions and changes in the form and details of thedevices illustrated, and in their operation, may be made by thoseskilled in the art without departing from the spirit of the presentinvention. For example, it is expressly intended that all combinationsof those elements and/or method steps which perform substantially thesame function in substantially the same way to achieve the same resultsare within the scope of the invention. Substitutions of elements fromone described embodiment to another are also fully intended andcontemplated. It is also to be understood that the drawings are notnecessarily drawn to scale but that they are merely conceptual innature. It is the intention, therefore, to be limited only as indicatedby the scope of the claims appended hereto.

What is claimed is:
 1. A slot die for producing webs of a foamedplastics material, comprising: a housing; and a pair of rollers arrangedin the housing so as to form a die gap through which plasticizedplastics material is channeled under such a pressure that foaming of theplastics material only occurs after it leaves the die gap, the pair ofrollers being mounted in a laterally sealed manner in the housing of theslot die so as to prevent a pressure drop in the plasticized plasticsmaterial before it leaves the die gap, the rollers of the pair ofrollers being frustoconical and displaceable axially with respect toeach other so that the thickness of the die gap is set.
 2. A slot die asdefined in claim 1, wherein the pair of rollers has two sides eachprovided with a sealing thread that acts as a melt seal and as a lateralsealing means which acts as one of a return conveying thread and asealing thread of an active melt seal.